Chlorosulfonyl isocyanate is industrially useful as an intermediate for producing pharmaceutical and agrochemical compounds, etc.
Conventionally, it is known that chlorosulfonyl isocyanate can be produced by the reaction of sulfur trioxide with cyanogen chloride, and several production methods thereof have been reported. For example, (a) Chem. Ber., 89, 1071 (1956) and West German Patent No. 928896 disclose a method in which sulfur trioxide is added to cyanogen chloride and reacted at a low temperature of −5° C. Also, (b) European Patent No. 294613 and Swiss Patent No. 680292A5 disclose a method for reacting sulfur trioxide with cyanogen chloride at 100 to 200° C.
However, the aforementioned method (a) is not preferable in view of cost. Moreover, there are problems with the quality, especially the purity, which does not meet commercial requirements. Furthermore, when the aforementioned method (b) is used, it is not easy to control the flow rate of sulfur trioxide and cyanogen chloride added to a reaction system, and the yield of the obtained chlorosulfonyl isocyanate is low and the quality thereof is poor, similar to when the method (a) is used.
In order to solve the aforementioned problems, (c) a method in which cyanogen chloride is added to sulfur trioxide and reacted while keeping the temperature of the reaction system at 20 to 50° C. (Japanese Unexamined Patent Application, First Publication No. S 63-77855), (d) a method in which sulfur trioxide and cyanogen chloride are added to a reaction system at the same time while keeping the temperature of the reaction system at 10 to 50° C. (Japanese Unexamined Patent Application, First Publication No. H1-228955), (e) a method in which sulfur trioxide and cyanogen chloride are reacted in a chlorinated hydrocarbon solvent (Japanese Unexamined Patent Application, First Publication No. H4-164063), and (f) a method in which cyanogen chloride is added to a mixture of sulfur trioxide and chlorosulfonyl isocyanate and reacted while keeping the temperature of the reaction system at −10 to 17° C. (Japanese Unexamined Patent Application, First Publication No. 2000-53630), and so forth have been proposed.
More recently, (g) a method has been proposed, in which a residue in a tank is decomposed after separating chlorosulfonyl isocyanate by distillation and, at the same time, a low-boiling-point fraction obtained by separating a chlorosulfonyl isocyanate by distillation or by decomposing/distilling the residue in the tank is recovered, and the recovered liquid is reused by adding when reacting sulfur trioxide and cyanogen chloride (Japanese Unexamined Patent Application, First Publication No. 2003-40854).
In those methods (c) to (g), chlorosulfonyl isocyanate having a comparably high yield (74 to 91%) and comparably high purity (90 to 98%) can be obtained by a comparably easy method.
However, in the methods (c) to (f), because a cooling operation is needed to maintain the temperature within a suitable temperature range in which the temperature may be raised due to an exothermic reaction with sulfur trioxide and cyanogen chloride and the cost for the cooling equipment is high, these methods are not satisfactory as an industrial method. Also, in order to improve the yield in those methods, it is essential to obtain chlorosulfonyl isocyanate by thermo-decomposing chloropyrosulfonyl isocyanate as a by-product. Because an operation in which chloropyrosulfonyl isocyanate is thermo-decomposed is dangerous, complex, and costly, these methods have industrial problems.
Moreover, in the method (f), the number of processes is large, and a complex after-treatment is needed for salvaging and reusing the low-boiling-point fraction, and thus the method (f) has a problem in that a lot of equipment is required.
In consideration of these circumstances surrounding the prior art, the object of the present invention is to provide a method for producing chlorosulfonyl isocyanate by a simple and easy operation, in which chlorosulfonyl isocyanate of high yield and high purity is obtained and the number of pieces of equipment is reduced.